1. Field of the Invention
The present invention relates to a signal receiving apparatus suitable for use in an in-vehicle apparatus of a remote keyless entry (RKE), and more specifically, to a signal receiving apparatus in which, when a receiving circuit board having an antenna pattern thereon is provided, the antenna pattern is formed such that circularly polarized radio wave signals can be received, thereby obtaining a small and light signal receiving apparatus.
2. Description of the Related Art
Conventionally, a remote keyless entry includes one or more portable transmitters that users individually keep and one or more in-vehicle receivers mounted on a vehicle. With control of any of the portable transmitters, radio signals are transmitted from the portable transmitter, and when the in-vehicle receiver receives the radio signal, the in-vehicle receiver checks whether the portable transmitter that transmitted an ID code included in the radio signal has already registered the ID code onto the in-vehicle receiver, and when it is checked that the ID code has been registered, the in-vehicle receiver supplies a control signal to a controlled unit of the vehicle indicated by a control code, using the control code included in the radio signal. For example, in a door lock apparatus, by providing a door unlock signal, the door lock apparatus is changed from a lock state to an unlock state.
In this case, the radio signal transmitted from the portable transmitter to the in-vehicle receiver is typically in a frequency range of 300 MHz to 500 MHz, and in addition, a transmission power of the radio signal is considerably small. To this end, for the in-vehicle receiver, a high sensitive receiving antenna for use in the in-vehicle receiver, in particular, a relatively large antenna is used so as to precisely receive the radio signal transmitted from the portable transmitter.
FIG. 3 is a plan view showing an example of the conventional antenna arrangement that can be used in this type of in-vehicle receiver, disclosed in Japanese Unexamined Patent Application Publication No. 11-355022, and FIG. 4 is an exploded perspective view showing another example of the conventional antenna arrangement that can be used in this type of in-vehicle receiver in the same manner, disclosed in Japanese Unexamined Patent Application Publication No. 06-117153.
As shown in FIG. 3, the antenna has a front chassis 31 in a thin box shape, a bottom chassis 32 in a thin box shape, a printed board 34 supported by supporting units 33 in a box space surrounded by the front chassis 31 and the bottom chassis 32, and a plate 35 fixedly attached at two spots 36 and 37 in an inverse L-shape with respect to the printed board 34, and the printed board 34 has an opening 38 in a portion facing the plate 35. Therefore, one spot 36 where the printed board 34 and the plate 35 are fixedly attached to each other forms an electrical connection terminal to the printed board 34, and the other spot 37 where the printed board 34 and the plate 35 are fixedly attached forms a grounded terminal. In the antenna arranged as described above, the plate 35 is used as an antenna element, so that an antenna having a relatively high sensitivity can be obtained which results in increasing the size thereof as a whole.
In addition, as shown in FIG. 4, the antenna is a pattern antenna 42 in which the length of the antenna mounted on the circuit board 41 is (¼) λ, and further, the circuit board 41 has a control circuit unit 43, an external capacitor 44, a high frequency input terminal 45, an antenna side wiring 46 that connects a pattern antenna 42 with one end of the capacitor 44, and a circuit unit side wiring 47 that connects the control circuit unit 43, the capacitor 44, and the high frequency input terminal 45. The antenna arranged as described above uses the pattern antenna 42 as an antenna element, so that the size can be reduced and thus the receiving signal directivity of the antenna is strong in one direction.
Recently, a remote keyless entry is proposed that acts as the remote keyless entry as described above as well as detects a tire pressure of a tire using a tire pressure sensor, supplies to a signal receiving apparatus a signal representing a detection result of the tire pressure as a radio signal, and processes the detection signal received by the signal receiving apparatus using the in-vehicle receiver, and thus is capable of detecting the tire pressure of the vehicle all the time.
The remote keyless entry proposed herein mounts on each tire of the vehicle the tire pressure sensor that detects an air pressure of the tire and a radio signal transmitter, respectively, mounts on the side of the vehicle signal receiving apparatuses that individually receive the respective radio signals, wirelessly transmits radio signals as detection signals detected by the respective tire pressure sensors from the corresponding radio signal transmitters, and allows the corresponding signal receiving apparatuses to receive the radio signals transmitted from the radio signal transmitters.
Therefore, the signal receiving apparatus described above requires an antenna that receives the radio signal in the same manner as in the in-vehicle receiver described above. Thus, the respective radio signal transmitters are mounted on rims of the corresponding tires, and the respective signal receiving apparatuses are mounted in corresponding wheel houses, so that a gap between the radio signal transmitter and the signal receiving apparatus is 10 to 15 cm in the nearest state to each other and 50 to 60 cm in the farthest state. To this end, although these signal receiving apparatuses do not need to use an antenna having high receiving sensitivity as disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 11-355022, if the antenna having high receiving sensitivity is used like this, the mounting spot is restricted when mounting the signal receiving apparatuses on the vehicle as well as it is difficult to use when the receiving sensitivity is too high. Therefore, it is difficult for the above-mentioned antenna to be used in this type of signal receiving apparatus.
In addition, for the signal receiving apparatus described above, since the position of a signal source of a radio signal received by the signal receiving apparatus, or the position of the radio signal transmitter is changed as the tire rotates, it is necessary that the antenna have the receiving directivity so as to receive the radio signal transmitted from the radio signal transmitter all the time even though the position of the radio signal transmitter is changed. By the way, when the antenna is used as disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 06-117153, the arrangement of the antenna is in a state where a receiving sensitivity for a horizontally polarized wave of the radio signal and a receiving sensitivity for a vertically polarized wave are significantly different, that is, a state where circularly polarized wave signals can not be received. For this reason, when the radio signal transmitter is placed in a predetermined range and direction with respect to the signal receiving apparatus, the receiving sensitivity of the antenna with respect to the radio signal transmitted from the radio signal transmitter is increased and thus the signal receiving apparatus may correctly receive the radio signal, however, in a moment that radio signal transmitter is out of the range with respect to the signal receiving apparatus, the receiving sensitivity of the antenna with respect to the radio signal transmitted from the radio signal transmitter is decreased and thus the signal receiving apparatus may not correctly receive the radio signal, so that it is difficult for the antenna described above to be used in this type of signal receiving apparatus.